Mercury is a highly toxic element, and globally its discharge into the environment is coming under increasingly strict controls. This is particularly true for power plants and waste incineration facilities. Almost all coal contains small amounts of speciated and elemental mercury along with transition metals (primarily iron) and halogens (primarily chlorine with small amounts of bromine).
Mercury in coal is vaporized in the combustion zone and exits the high temperature region of the boiler entirely as HgO while the stable forms of halogens are acid gases, namely HCl and HBr. The majority of coal chlorine and bromine form HCl and HBr, respectively, in the flue gas since the formation of elemental chlorine and bromine are limited due to other dominant flue gas species including water vapor, sulfur dioxide (SO2), nitrogen oxides (NOx) and sulfur trioxide (SO3). By way of example, the Griffin reaction holds that sulfur dioxide, at the boiler temperature range, reacts with elemental chlorine to form sulfur trioxide and HCl. Elemental mercury oxidation primarily to mercuric chloride and bromide species occurs via both homogeneous gas-phase and heterogeneous reactions that involve HCl and HBr, respectively. For low rank coals with low to medium sulfur and low chlorine and bromine contents however, homogeneous gas-phase Hg oxidation reactions are believed to be limited primarily by elemental Cl2 and Br2 rather than by HCl and HBr due to the slow reaction rate of HCl and HBr. Therefore, though homogeneous gas phase mercury oxidation by elemental chlorine does occur as the flue gas cools it is not the dominant reaction pathway because insufficient elemental chlorine is generally present. Rather, heterogeneous reactions controlled by HCl in the cooler regions of the flue gas path past the economizer section and especially occurring within and downstream of the air preheater and on duct surfaces are considered to be the primary reaction pathway for oxidation of elemental mercury by chlorine. At cooler flue gas temperatures elemental halogens may be formed from HCl and HBr by a Deacon process reaction. HCl and HBr react with molecular oxygen at cooler flue gas temperatures to form water and elemental chlorine and bromine, respectively. This reaction is thermodynamically favorable but proceeds only in the presence of metal catalysts that are primarily present on the surface of entrained fly ash particles or on duct surfaces.
The U.S. Geological Survey database COALQUAL gives halogen data from analyzed coal specimens. According to this data, U.S. coals have bromine contents between 0 and 160 ppm and the mean and median bromine concentration of the coals are 19 and 12 ppm, respectively, and chlorine contents between 0 and 4,300 ppm and the mean and median chlorine concentration of the coals are 569 and 260 ppm, respectively. Lignite and sub-bituminous (e.g., Powder River Basin (“PRB”)) coals are significantly deficient in halogens as compared to average U.S. coals while bituminous coals are higher in halogens than the lower rank coals. For lower rank coals, Hg° is the predominant vapor mercury species.
Various methods of augmenting HCl to increase oxidized mercury have been tested at full-scale. Direct addition of halide salts to the coal or injection of halide salts into the boiler has been attempted. There have also been a number of trials of coal blending of low-rank subbituminous coals with higher chlorine bituminous coals. Increased chlorine in the boiler in the form of halide salts or higher chlorine results in an increase of primarily HCl in the flue gas and very limited Cl2. These tests appear to indicate that excess HCl alone does not significantly increase the HgCl2++ fraction unless a mechanism exists to make Cl available. Naturally occurring mechanisms that appear to be effective include catalysts in the form of activated carbon or LOI carbon.
For lower rank coals, there is thus a need for an effective mercury control methodology.